Polyethers and polyether siloxanes are often used as an additive for producing, or for printing on, foodstuffs packaging. In this sector in particular, the substances employed are subject to very strict requirements in terms of their potential to migrate into the foodstuff. To avoid absorption of such substances into the human body, said substances must have little, if any, migration potential. Great emphasis is therefore placed, even during synthesis of the basic polyether components, on using starting materials, particularly alcoholic starters, that are toxicologically uncritical. It is of particular importance that the starting materials are non-toxic in small amounts and exhibit only little migration into the foodstuffs in case said starting materials do not undergo complete conversion in the reaction to afford the polyether and are therefore still present in the product.
It is therefore an object of the present invention to provide polyethers which meet the tight targets for additives in contact with foodstuffs and, simultaneously, are stable, homogeneous compounds bearing reactive groups allowing optional further reaction to afford the polyether siloxane.
However, many of the alcoholic starters typically employed in the DMC-catalysed production of polyethers meet the criterion of foodstuffs conformity and little migration only to a limited extent, if at all.
It has now been found that, surprisingly, the use of eugenol as starter in the DMC-catalysed production of polyethers affords products that are particularly homogeneous, have a small molar mass distribution and are unusually stable. Assessment of stability must in particular take account of the fact that using eugenol as starter results in a polyether whose terminal double bond is almost entirely preserved, i.e., is not subject to rearrangement to form isoeugenol. This has enormous advantages since the preservation of the terminal double bond makes countless further chemical reactions possible, in particular hydrosilylation with Si—H-functional siloxanes.
Surfactant polyethers based on aromatic alcohols, i.e. phenols, as starters are sufficiently well-known as described in U.S. Pat. No. 5,296,627 A and U.S. Pat. No. 6,646,091 B2 for example.
The base-catalysed alkoxylation of eugenol is also known in principle. For instance, Moustapha et al. refer in Egyptian J. Chem. 2005, 48 (3), 273-285 to the sodium metal-catalysed ethoxylation of eugenol. The product is not characterized in detail and is used merely as solvent for silver-alkene complexes in gas chromatographic analyses.
Documents EP 94386 B1 and DE 3342509 A1 describe compositions comprising eugenol-based polyethers.
The first detailed description of the alkali-catalysed alkoxylation of eugenol is described in EP 1717259 A1. In the examples reported therein, eugenol is initially charged as starter and then admixed with an alkaline catalyst such as sodium methoxide. After removal of the methanol from this catalysis step, ethylene oxide, propylene oxide and/or butylene oxide are added on at temperatures of 140-160° C. This procedure demonstrably affords pure isoeugenol-based polyethers, i.e. the eugenol allyl group undergoes quantitative rearrangement to form a 2-propenyl group during the alkaline alkoxylation. The resulting structural unit is known to those skilled in the art as isoeugenol.
In Macromol. Symp. 2010, 293, 15-19, Luinstra et al. also describe polyether-like structures where the eugenol allyl group remains stable in order to subject said structures to an ADMET polymerization. Said authors employ a substitution reaction of diethylene glycol ditosylate with two mol of eugenol. However, a eugenol-containing polymer is not described.
Polyether siloxanes bearing eugenol groups are also disclosed in principle in the scientific literature and can be obtained by three synthetic principles.
Patent application JP 11158266 A and granted patent EP 2134771 B1 describe the incorporation of eugenol into the polyether siloxane backbone by equilibration of eugenol-capped polysiloxane with hydrogen-bearing cyclic hydrosiloxanes such as D4H for example.
Patent U.S. Pat. No. 6,313,329 B1 discloses a particularly elegant method of introducing a eugenol unit into a polyether siloxane structure. This comprises initial hydrosilylation of a conventional terminally unsaturated polyether, along with methyl undecylenate, onto a SiH-bearing polysiloxane under Pt catalysis and subsequent transesterification of the polysiloxane-bonded methyl ester with the phenolic oxygen to eliminate methanol. However, the instability of the resulting phenol ester in aqueous systems that is to be expected will presumably limit the commercial utility of such products severely.
The method most commonly used, on account of it being the most advantageous to implement industrially, comprises linking the eugenol allyl group to Si—H-functional polysiloxanes by hydrosilylation, generally under catalysis by Pt compounds. For instance, granted patent EP 818495 B1 describes triazine-functional polyether siloxanes also comprising eugenol units for permanent finishing of textiles and leather.
In most cases, not only eugenol but also conventional terminally unsaturated polyethers, for example alkoxylates of allyl alcohols, are hydrosilylated onto the SiH-bearing alkylpolysiloxanes, in individual cases also with further terminally unsaturated compounds such as alkenes for example. As described in EP 1010748 B1, EP 887367 A3 and EP 845520 B1, such eugenol-comprising polyether siloxanes are used as diesel defoamers.
Cosmetic formulations constitute a further broad field of application where such polyether siloxanes are used as adjuvants, as described in EP 1260552 B1, U.S. 63/466,595 B1, EP 916689 B1 and U.S. Pat. No. 7,287,784 B2.
2011 also saw the description of polyglycerol-containing polyether siloxanes which comprise polymer-bonded eugenol units at least to some extent. Applications EP 2492301 A1 and EP 2492333 A1 describe the hydrosilylation of polyglycerol allyl ethers, resulting from eugenol-glycidol adducts, onto SiH-bearing polyether siloxanes. Such hydrophilic polysiloxanes may be used as thickeners or emulsifiers.
The disadvantage, in process engineering terms, of the prior art method of producing polyether siloxanes bearing eugenol groups and polyether groups by hydrosilylation is that two or more unsaturated products must be added onto the SiH-bearing polyether siloxane simultaneously. The significant differences in terms of both molecular weight and hydrophilic/hydrophobic character between the reactants such as eugenol and the polyether(s) to be added on (and naturally the SiH-bearing polysiloxane too) impede the production of a polyether siloxane of uniform composition in which the different reactants are evenly distributed over all siloxane chains. Inadequate commixing very rapidly results in products of inhomogeneous composition and it is imperative that this be avoided on quality and cost grounds.
The use of solvents to homogenize the reaction medium is certainly conceivable but disadvantageous in terms of cost and process engineering since the added solvent then needs to be removed again in a further process step after hydrosilylation.
It therefore appeared useful to attempt to combine the aromatic character of the hydrosilylatable eugenol with the tunable hydrophilic/hydrophobic character of a polyether in one molecule/polymer and subsequently to subject said molecule/polymer to a hydrosilylation reaction with a suitable polyether siloxane without using a solvent.
Since the alkali-catalyzed alkoxylation of eugenol demonstrably affords isoeugenol-based polyethers (vide infra), such polyethers cannot be hydrosilylated onto SiH-bearing polysiloxanes. It is common knowledge that 2-propenyl groups are not amenable to hydrosilylation.